I just spray the underside and rotors with pressurized water. I tried to do that at least once a week during winter time and once a month during summer time.
We have a great car wash here that has an undercarriage spray that sprays up from ground as you drive in. I usually drive back and forth over it a few times and it seems to work great.
haven't done this myself. I've read if the underside is really muddy just put a lawn sprinkler under the vehicle to wash away the mud. Sounds like it would work.